A DIGITECH FS3X MIDI FOOT SWITCH CONTROLLER will control the first Eventide stompbox in your midi chain – tap tampo by using the FS3X's left-hand switch.  You set up your chain of Eventides to accept this and pass it on to the next one, and you'll be able to tap tempo for all three (or four if you've got them!) boxes.

 (You set each stompbox to a different channel – as already explained above – to select different presets.)

 You need to be using the latest Eventide software update…. IMPORTANT

The EFX Mk3 allows you to transmit midi parameters from an Expression Pedal to all midi devices – – see page 9 of the handbook , or you can have separate expression Pedals plugged into each Eventide.

If using one plugged into the EFX Mk3, the Ex Pedals' channel and number from the EFX are global, so you have to tell each Eventide what  it is, and each will then do whatever you've programmed it to do  whenever it receives cc value changes. You could try programming it with the EX pedal plugged into an Eventide to make it simpler, or one stomp box at a time.